The field of the invention is removal of a gaseous component from a process gas.
Various methods are known in the art to remove a gaseous component from a stream of a process gas, including a wide range of distillation-, adsorption-and absorption processes, and one relatively common process involves regenerator-absorber systems.
In a typical regenerator-absorber system, gas is introduced in the absorber where the gas contacts a lean solvent traveling down the column. The gaseous component is at least partially absorbed by the lean solvent, and the purified process gas leaves the absorber for further processing or discharge. The lean solvent containing the gaseous component (i.e. the rich solvent) flows through a cross heat exchanger thereby increasing its temperature. The heated rich solvent is then stripped at low pressure in a regenerator. The stripped solvent (i.e. lean solvent) is sent back through the cross heat exchanger to reduce the temperature in the lean solvent before completing the loop back to the absorber. The regenerator-absorber system process typically allows continuous operation of removal of a gaseous compound from a process gas at relatively low cost. However, the efficiency of removal of the gaseous component is not always satisfactory, and especially when the gaseous component is carbon dioxide, stringent emission standards can often not be achieved with a standard regenerator-absorber system. To overcome problems with low efficiency the temperature or pressure in the regenerator may be increased. However, corrosivity and solvent degradation generally limit the degree of optimization for this process.
An improved regenerator-absorber system is shown by Shoeld in U.S. Pat. No. 1,971,798 that comprises a split-loop absorption cycle, in which the bulk of the solvent is removed from an intermediate stage of the regenerator column and recycled to an intermediate stage of the absorber. In this arrangement only a small portion of the solvent is stripped to the lowest concentration, and a high vapor to liquid ratio for stripping is achieved in the bottom trays of the absorber, resulting in somewhat lower energy use at low outlet concentrations. However, the reduction in energy use is relatively low due to thermodynamic inefficiencies in stripping, mainly because of variations in the solvent composition as it circulates within the split loop.
To circumvent at least some of the problems with the split loop process, various improvements have been made. For example, one improvement to the split-loop process is to more accurately control the concentration of solvents. To more accurately control the solvent concentrations, two modifications are generally necessary. The first modification comprises an intermediate reboiler, which is installed to a main regenerator to boil off water from the semi-lean solvent to adjust the concentration of the semi-lean solvent stream to the concentration of the lean solvent. The second modification comprises a side-regenerator to regenerate condensate from the main regenerator. The condensate from the main regenerator is sent to the top section of the main regenerator, where it undergoes partial stripping, and is then further stripped to a very low concentration of dissolved gas in the side-regenerator, before being returned to the bottom reboiler of the main regenerator.
Since only a relatively small portion of the total solvent (typically xcx9c20%) is stripped to the ultra-low concentration, the process allows achieving relatively low outlet concentrations with comparably low energy use. Furthermore, when methyl diethanolarnine (MDEA) is employed as a solvent in the improved split-loop process, the liquid circulation can be reduced by about 20%. However, the modifications to improve energy use and lower solvent circulation generally require a substantial modification in the configuration of the main regenerator, and the installation of a side-regenerator, both of which may result in substantial costs and significant down-time of an existing absorber-regenerator system.
Another improvement to the split-loop process is described by Shethna and Towler [xe2x80x9cGas Sweetening to Ultra-low Concentrations using Alkanolamines Absorptionxe2x80x9d; Paper 46f, AlChE Spring Meeting, N.O. 1996], in which two regenerator columns are utilized. A primary regenerator produces a semi-lean solvent, and a secondary regenerator produces an ultra-lean solvent. A small portion of the purified process gas leaving the absorber is expanded to a lower pressure level thereby producing a cooled purified process gas. The heated ultra-lean solvent stream leaving the secondary regenerator is cooled by the cooled purified process gas thereby producing a heated purified process gas, which is subsequently fed into the secondary regenerator. The recycled gas is then recovered from the secondary regenerator and reintroduced into the feed gas stream at the absorber.
The use of a substitute vapor instead of a reboiled solvent at the secondary regenerator advantageously lowers the partial pressure of the solvent vapor in the secondary regenerator, and allows the secondary regenerator to operate a lower temperature than the primary regenerator column. Operating the secondary regenerator at a reduced temperature typically results in a reduced corrosivity of the solvent, which in turn may allow for the use of cheaper materials such as carbon steel in place of the conventional stainless steel. Furthermore, a split-loop process using vapor substitution may be combined with fixed-bed irreversible absorption technology, e.g. to remove H2S and or COS from the recycle gas in a bed of solid sorbent, thereby ensuring a relatively long bed life of the absorber. However, the split-loop process using vapor substitution requires the use of least two regenerator columns, and it may further be necessary to re-tray the top stages of an existing absorber to accommodate for the needs of this particular process. Moreover, due to the recycle gas and the use of a secondary regenerator column, retrofitting of existing absorber-regenerator combinations may be relatively expensive and time consuming.
Although various improvements to the general layout of a absorber-regenerator process have been known in the art, all or almost all of them suffer from one or more than one disadvantage. Therefore, there is a need to provide improved methods and apparatus for the removal of a gaseous component from process gases.
The present invention is directed to a recovery plant to recover a gaseous component from a process gas, having an absorber that employs a lean solvent and a semi-lean solvent which absorb the gaseous component from the process gas, thereby producing a rich solvent, a semi-rich solvent, and a lean process gas. A regenerator is coupled to the absorber, wherein the regenerator extracts the gaseous component from the rich solvent, thereby regenerating the lean solvent and the semi-lean solvent. A solvent flow control element is coupled to the absorber and combines at least part of the semi-rich solvent with at least part of the semi-lean solvent to form a mixed solvent. A cooler is coupled to the absorber that cools the mixed solvent, and the cooled mixed solvent is subsequently fed into the absorber via a connecting element.
In one aspect of the inventive subject matter, the process gas is a flue gas from a combustion turbine, having a pressure of less than 20 psia when fed into the absorber, and wherein the gaseous component is carbon dioxide. The concentration of carbon dioxide is preferably greater than 2 mole %, more preferably greater than 5 mole %, and most preferably greater than 10 mole %.
In another aspect of the inventive subject matter, the solvent comprises a chemical solvent, preferably selected from the group consisting of monoethanolarnine, diethanolamine, diglycolamine, and methyldiethanolamine. It is also preferred that appropriate solvents have a concave equilibrium curve.
In a further aspect of the inventive subject matter, a method of removing a gaseous component from a process gas has a first step in which a stream of lean solvent and a stream of semi-lean solvent is provided. In a second step, the process gas is contacted with the stream of lean solvent and semi-lean solvent in an absorber to produce a stream of semi-rich solvent and a stream of rich solvent. In a further step, at least part of the semi-rich solvent and at least part of the semi-lean solvent are combined to form a mixed solvent stream, in a still further step the mixed solvent stream is cooled and the cooled mixed solvent stream is introduced into the absorber to absorb the gaseous component.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.